People who are jobless at some point during their lifetime because they were laid off, fired or quit may be at an increased risk of having a heart attack after age 50, finds a study out Monday.

In fact, the chances of a heart attack associated with multiple job losses may be on par with the risks people face from factors such as smoking, hypertension and diabetes, says the study's lead author, Matthew Dupre, an assistant professor of medicine at Duke University.

The researchers say they don't know from the data if the job losses were because people were fired, laid off, had seasonal jobs or voluntarily left their jobs. "We believe the greatest risk for heart attacks would come from having been fired or laid off -- in other words involuntary job loss," says Linda George, a professor of sociology at Duke and an author on the study.

"We do know it's not from retirement," George says. "Retirement poses no increased risk of heart attack."

Researchers at Duke looked at the different aspects of unemployment and the risks of heart attacks among 13,451 men and women, ages 51 to 75, who participated in the national Health and Retirement Study. Participants were interviewed every two years from 1992 to 2010.

Using statistical models, the researchers looked at associations between multiple aspects of employment instability and heart attacks. Among the findings presented online Monday in the Archives of Internal Medicine:

Heart attack risks were about 35% higher among the unemployed than employed, and risks increased incrementally from one job loss (22% higher) to four or more job losses (63% higher), compared with those without a job loss in their lifetime.

The risk of having a heart attack was highest the first year of unemployment.

The harmful effects of unemployment were consistent for men and women, and major race/ethic groups.

Participants had the same risk of a heart attack from unemployment no matter what their education level or socioeconomic situation, Dupre says.

George says the researchers don't know the exact mechanisms for the increased risk, but they do know that "anytime we are not as in control of our lives as we'd like to be, stress goes up."

When that happens, other health habits may slide too — people may eat less healthfully, stay up too late and not sleep as well, she says. There may be more strain and conflicts in the family. "We believe all these things are among the reasons why unemployment is linked to this increased risk of heart attack."

People should "be extra vigilant" about seeking medical help during times of unemployment, especially if they are experiencing any signs of a heart attack, George says.

Atlanta cardiologist Gina Lundberg, an assistant professor of medicine at Emory University who was not part of this study, says, "Research shows that job stress can cause heart attacks, and now this study shows that not having a job causes heart attacks.

"Right now, many Americans have stressful jobs or no job at all — and either way, it isn't good for their heart."